Electrode drive control circuit



June 21, 1960 v. H. CARR ETAL 2,942,138

ELECTRODE DRIVE CONTROL cmcurr Filed June 29, 1959 vi i IN VEN TORSVICTOR H. CARR DAVID J- KLEE BY LOUIS KRIVANEK UUQDOW A TTORNE Y UnitedStates Patent ELECTRODE DRIVE CONTROL CIRCUIT Victor H. Carr, David J.Klee, and Louis Krlvanek,

Shelbyvllle, Ind., assiguors to General Electric Company, a corporationof New York Filed June 29, 1959, Ser. No. 823,375 11 Claims. (Cl.314-69) The present invention relates to electrode drive controlcircuits, and more particularly to control circuits for the electrodedrive of a consumable electrode type electric arc furnace.

In an electric arc furnace of the consumable electrode type, the heatfor the furnace is provided by an electric arc between a molten pool ofmetal, that serves as one electrode for the arc, and a second electrodethat is movable relative to the molten pool of metal. The secondelectrode is consumed by the heat of the arc, so for proper operation ofsuch a furnace, which, among other things, requires a constant. arcvoltage to" be applied between the two electrodes, the position of thesecond electrode must be adjusted relative to the molten pool tocompensate for the consumption of the electrode. The metallurgicalcharacteristics of the molten pool of metal, and consequently of theingot formed, are affected if the distance between the second electrodeand the molten pool is not accurately controlled. In particular, if thedistance between the second electrode and the pool is allowed to becometoo large, a glow discharge condition may be established which may makethe resultant ingot metallurgically unacceptable if the condition is notof the pool toward the wall of the crucible with the attendantpossibility of a burn-through of the crucible wall. On the other hand,if the arc gap becomes so small that the movable electrode touches the,pool, the electrode may become welded to the molten pool, as the metaltherein cools due to the are being extinguished bysuch contact. 7 Oneknown method of providing an accurate automatic control of electrodemovement is to connect a control circuit that is responsive to the arcvoltage to an electrode drive mechanism so that the electrodes aredriven apart if the arc voltage falls below a predetermined level anddriven toward one another if the arc voltage rises above a predeterminedlevel. While such a control circuit is generally satisfactory for normalfurnace operation, it

does not perform certain functions which it is desirable to provide forpurposes of ease and safety of operation. For example, when a glowdischarge condition occurs between the electrodes, the arc voltage dropsand thus causes such a drive mechanism to drive the electrodes apart, orat least stop them from being driven toward "ice destructive magnitudebefore it is detected. Consequently, manufacturers have been confrontedwith the problem that an ingot formed during a manufacturing run inwhich a glow discharge condition occurs has often been metallurgicallyunacceptable.

A solution to this problem is provided by our invention which providesmeans for detecting a glow discharge condition in an arc furnace withina very short interval of time after such a condition has started;therefore, the furnace operator is alerted to take corrective measuresbefore the condition can grow to a sufficient magnitude to damage theingot.

Accordingly, it is a specific object of our invention to provide in anautomatic electrode drive control system of the type which is responsiveto relative movement between the two electrodes of an arc furnace, animproved control circuit whereby a glow discharge condition is detectedwithin a predetermined time interval such that harmful effects resultingfrom the glow discharge condition are prevented.

It is another object of our invention to provide, in a control system ofthe type just described, an improved arrangement for continuouslymonitoring the relative movement between two electrodes and forenergizing a signal if the relative movement varies from a predetermineddesirable pattern of operation.

- It isa further object of our invention to provide a drive controlcircuit for relatively movable members, which has time delay signalactuating means responsive to a deviation from the normal operatingcondition of the movable member drive to energize a signal which informs an operator of the abnormal condition.

Further objects and advantages of our invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize our invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

In carrying out the objects of our invention in one form thereof, thereis provided a basic type of automatic control circuit for an electrodedrive, which is known in the prior art and which generally comprises anamplidyne, the armature of which is connected to a reversible directcurrent motor, which in turn is drivingly coupled to one of a pair ofelectrodes. The field current of the amplidyne is responsive tovariations between a fixed reference voltage and the arc voltage betweenthe pair of electrodes so that when the reference voltage is higher thanthe arc voltage, the motor will be energized by the amplidyne to efiectseparation of the electrodes, while if the arc voltage is higher thanthe reference voltage, the motor will drive the electrodes toward oneanother.

To the basic electrode drive control circuit is added a circuit whichprovides means for detecting a glow discharge condition, and means forsignalling an operator within a predetermined timed interval of shortduration, while at the same time allowing the basic drive controlcircuit to effect frequent stoppages or reversals in the direction ofthe relative movement of the electrodes, without causing the signalmeans to be sporadically actuated by such normal operation.

Briefly stated, to the basic electrode drive control circuit has beenadded a circuit that detects either a reversal in the direction ofrelative motion between the pair of electrodes (assuming the electrodesare moving toward one another), or a stoppage of relative motion betweenthe electrodes, such as will occur if a glow discharge condition existsbetween the electrodes. This motion detecting circuit is connected toanother circuit containing time delay means for actuating a signal. Inoperation, the motion detecting circuit responds to a stoppage orreversal in the direction of-the relative movement of the electrodes byactuating the time delay circuit,

The time delay circuit, when thus actuated, initiates a timed signalactuating cycle of predetermined duration. It no further actuation isreceived by the time delay circuit from the motion detecting circuit,the signal means is actuated by the time delay circuit at the end of thepredetermined timed interval to inform an operator .of .the abnormalelectrode movement and the possible glow discharge condition of which itmay be indicative. If, however, prior to the expiration of "thepredetermined timed interval, the electrodes again move toward oneanother, the motion detecting-circuit will respond to this movement bycausing the time delay circuit to terminate 2 and actuates time delaysignally circuit 5 in response to such variations, comprises a voltageresponsive relay 18 electrically connected in series with aunidirectional current passing means, shown as a rectifier 19, acrossthe output of amplidyne 7. This circuit arrangement causes relay 18 tobe energized when the output voltage of amplidyne 7 is of one polarity,whilecausing relay 18 to be dethe timed signal actuating cycle and resetthe timing mechanism. I

For a complete understanding of theinvention, together with objects andadvantages thereof, reference is made to the following description,taken in conjunction with the accompanying drawing which depicts inschematic r :shown an electrode motion detecting circuit 4, and a :timedelay signal actuating circuit 5.

These added circuits, 4 and 5, operate in conjunction to detect anabenergized when {the eutputvoltage of amplidyne 7 is zero or of theopposite polarity. It is thus apparent, since the direction :of rotationof motor 6, and conse quently the movement or direction of movement ofelectrode 2, is also dependent-upon the magnitude or polarity of theoutput voltage of amplidyne 7, that the energization of relay 18 is adirect function of the movement of electrode 2.

In addition to these components of motion detecting circuit 4, namely,rectifier 19 and voltage responsive relay .18, this circuit includes.thenormallyopen contacts 21 of relay The contacts .21 :are in serieswith these com? ,ponents v(1.8 and 119) to prevent .high voltage surgesacross these components in 'a manner that will be more f 1l1 l ydescribed below. lniorder to regulate the sensitivity of relay 18 tovariations of the output voltage of amplidyne 7, there .is arranged inseries with rectifier 19 and normal rate of movement between electrode 2and crucible 3, such as will result if a glow discharge-condition occursbetween these elements, and to signal an operator if such ,a conditionprevails for a predetermined interval of time.

The illustrated basic electrode drive control circuit 1 comprises areversible direct current motor 6 drivingly coupled to the'e'lectrode 2and energized by an amplidyne 7. The primary amplidyne control field 8,which determines the output voltage polarity of amplidyne 7, andconsequently the direction of rotation of motor 6, is energized by botha direct current source 9 and the output of a bridge rectifier 10. Theinput terminals of rectifier 10 are supplied through auto-transformer 11from an alternating currentsource 12 of reference voltage. The relativemagnitude of the reference voltage, as adjusted by auto-transformer 11,and the voltage of the direct current source ,9 determines the directionof current through amplidyne field .8. In addition to the foregoingcomponents, the basic drive control circuit comprises means forelectrically connecting one side of control field 8 through a variableresistor 13, a pair of, series-connected normally closedelectrodemovement limit switches 14 and 15, a line 16, and rectifier 10to one side. of power source 9. The other side of control field .8 iselectrically connected through automatic-manual switch 17 (17 is shownon the drawing as having an auto and a man position) to the other sideof direct current source 9. Direct current source 9 in addition tosupplying control field 8,, provides the arc voltage to form an arebetween electrode 2 and crucible 3, or. the contents thereof.

In operation, the basic electrode drive control circuit 1 is utilized tomaintain a predetermined spacing between electrode 2 and crucible 3 byadjusting auto-transiormer 11 to a reference voltage consistent withsuch spacing. After the circuit is thus oriented, if the arc voltage becomes larger than the reference voltage, current through the amplidynefield 8 will be in a direction to eifect energization of the motor 6 inan electrode lowering ,direction. Conversely, when the arc voltage islowerthan the reference voltage, current passes through thcfield 8 inthe opposite direction to efiect raising of the electrode -2. Thus, anautomatic adjustment .of elect-rode position relay18 a variable currentlimiting resistor 22 and .a portion of the resistance of potentiometer23. The remains [mg portion or the resistance of potentiometer 23 isplaced in parallel with relay v18.. The movable contact of potentiometer23 is connected to one side of the actuating coil .of relay 18 and the.otherside of the vactuating coil :of relay 1. i s connected to thefixed terminal of pjotena tiorneter 23 that is common with one end ofvariable resistor .22. A coarse adjustment of the sensitivity of relay18 to variations in the output voltage oftamplidyne 7 is afforded .byvarying the value of resistor 22. For example, by reducing the value ofresistance 22 in series :withrelay 18,-.a. greater sensitivity of therelay is ob.- tained. .Afiner adjustment of the sensitivity of relay 18is obtained by varying the value of potentiometer 23. for example, toincrease the sensitivityof relay 18 to variations in the output voltageof amplidyne '7, the resistance. otpotentiometer 23 that is placed inparallel with the. actuating coil .of relay L8 is increased.

The time delay signalling circuit 5, which is responsive to theenergization of relay 18 and, therefore, to variations in the relativemovement .of electrode 2, serves ,to signal operator an abnormalvariation .in relative electrode movement continues .for [apredetermined length (If-time, The. time delay circuit 5 is madedirectly responsive .to the motion detecting, circuit .4 by arrangingnormally open contacts 24 of relay 18 in series with he ating-9 1 Qt t m-d e av 15- Th s c ponents, 2.4 and 25, are connected to opposite sidesof a power source 25 by lines 26 and,2,7 respectively. Relay 2.5 is .of:the time closing type, so its armature 29a be hiasedto the closedposition on contacts 29 when the actuatingcoil of relay Z5dea-energized, and the armsture 29a will be held open when the actuatingcoil is energized. vInorder to provide an indication of abnormal.electrode movement, a, signal. means, shown as a lamp 3,0, iaarranged inseries with contacts 29 and these .coxnponents, 29 and .30,areconnect'ed by lines 31 and dlrespcotivel y toopposite sides of powersource 28.

It will be apparent that when relay 18 is energized toiclose contacts 24thewactuating coil of time delay relay .25 willv beenergized to opencontacts 29..by moving armature 29a away from contacts v29 so that, theis p 3.0, decnergized'. Now, if relay 1:; is tie-ener- ;gizedvdue. to avariation in the voltage. of amplidyne 7, as described above, contacts24 will open and relay 25 dQeQBQISiZQd. rela is deaenergized, armature2.2a 'will'jstart rev-close contacts '2 bu S n e relay 25" is a 1imeclosing relay=,.this..closing cycle takes a predetermined length oftimev dependent tuponrtthc initialsettingioct the tirnin'g'r'neans (not.shown.) of relay 3 25. If relay 18 is not re-energized before theexpiration of the predetermined interval of time required for armalure29a to close contacts 29, lamp 30 will be energized by the closing ofcontacts 29 and the operator will thus be informed of the abnormalelectrode movement. However, if relay 18 is re-energized prior to theexpiration of the time closing interval, contacts 24 will be reclosedand relay 25 will be re-energized thus resetting the time closing cycleof armature 29a to contacts 29 so when relay 25 is again de-energized,the entire predetermined interval of time will have to expire beforelamp 30 is energized. During normal operation of the furnace, the signallamp vwill never be lit because electrode 2 will always move towardcrucible 3 at leastonce during the interval of time required forsolenoid armature 29a to close contacts 29, and thus continually reset.armature 29a so it never engages contacts 29.

The function of relay 33 is to properly orient the time delay circuit 5when switch 17a is moved from its auto to its man position. (It will beunderstood that the schematically indicated switches 17 and 17a areintended to be parts of a single control switch so they open and closein unison.) The actuating coil of relay 33 is connected by line 34 toone side of source 28 and by line 35 through auto position of switch 17ato the opposite side of source 28. Normally closed contacts 36 -of relay33 are arranged in parallel with normally open contacts 24 of relay 18so that when switch 17a is moved from its auto to its man" position, theactuating coil of time delay relay 25 will be energized through contacts36 to effect opening ofcontacts 29 and the deenergization of lamp 30.The time delay signal circuit 5 is thus reset for another cycle ofautomatic operation when switch 17a is moved from its man" to its auto"position.

As mentioned above, the normally open contacts 21 of relay 20 are inseries with the rectifier 19 and the actuating coil of relay 18 of themotion detecting circuit 4. The actuating coil of relay 20 is connectedby line 37 to one side of power source 28, and by line 38 through theauto position of switch 17a to the other side of power source 28. Byplacing contacts 21 of relay 20 in series with the actuating coil ofrelay 18 and rectifier 19, the circuit across the output of amplidyne 7is interrupted when switch 17a is in its man position. Such a circuitinterruption is desirable, because manual adjustment of electrode 2 mayresult in high voltages across amplidyne 7 that would damage eitherrelay 18 or rectifier 19.

The overall operation of the basic electrode drive-con trol circuit, themotion detecting circuit, and the time delay signalling circuit is asfollows:

-To initiate operation of the electrode drive control circuit to effecta melting operation in the crucible 3, switch -17--17a is placed in itsman position and power sources 9, 12, and 28 are energized. Switch17-17a is then moved to its auto position which, since the electrode 2is out of contact with the contents of crucible 3 and an arc has notbeen initiated, will cause current through amplidyne field 8 in such adirection that the amplidyne 7 energizes the drive motor 6 to lowerelectrode 2 toward crucible 3. Electrode 2 will be driven towardcrucible 3 until it contacts the contents of the crucible 3, because,until this occurs, no arc is formed between electrode 2 and the contentsof crucible 3. The voltage between these members (2 and 3) is muchhigher than the reference voltage of source 12 prior to the formation ofan arc. When electrode 2 contacts the contents of crucible 3, the arcvoltage drops to zero and the motor 6 is reversed, due to the reversalof current through amplidyne field 8. Motor 6 raises the electrode 2away from the crucible 3 until the arc voltage equals the referencevoltage and then drives the electrode 2 toward the crucible 3 at a ratethat compensates for the :closing of the contacts 29 of time delay relay25.

In normal operation of the arc furnace, the electrode 2 will move towardthe crucible 3 at a rate commensurate with the rate of consumption ofthe electrode 2. During such normal operation, although the direction ofmovement of electrode 2 is predominantly toward crucible 3, theelectrode movement will be stopped frequently and may even be reversedfor short intervals of time. The voltage across the armature ofamplidyne 7 will, therefore, be such that either a continuous current orclosely spaced pulses of current will be passed through rectifier 19 andthe actuating coil of relay 18. Therefore, due to the adjustment ofpotentiometer 23, contacts 24 will be either continuously closed orclosed at frequent closely spaced intervals to energize the time delayrelay 2-5 and prevent it from closing contacts 29. However, if a glowdischarge condition occurs between the electrode 2 and either thecontents of crucible 3 or the contents and the sides of crucible 3, thevoltage between these members will drop to such a level that currentthrough control field 8 of amplidyne 7 will either cease or thedirection of current through field 8 will be reversed. Either of theseconditions will cause a voltage to appear across the armature ofamplidyne 7 such that rectifier 19 will prevent any current through theactuating coil of relay 18; therefore, this relay will open contacts 24and thus de-energize the coil of the time delay relay 25 and initiate atimed closing interval of contacts 29. If the glow discharge conditioncontinues, the electrode 2 will not move toward the crucible 3 to causea reversal of current through control field 8 of amplidyne 7, so theactuating coil of relay 18 will not be energized during the timedinterval required for armature 29a to close contacts 29, and the signallamp 30 will be energized through contacts 29 and lines 31 and 32 tothus inform an operator that the furnace is operating in an abnormalcondition.

To correct the abnormal condition, the operator switches switch 17 17ato its man position and manually moves the electrode toward the crucible3 a sufii- ,cient distance to re-esta'blish anormal are betweenelectrode 2-and the contents of crucible 3. Due to the fact that theglow discharge condition is detected in its early tents of crucible 3;therefore, switch 17-17a is not provided with contacts that interruptthe arc voltage. When switch 17-'17a is moved from its auto position toits man position following the energization of signal lamp 30, theactuating coil of relay 33 is de-energized and normally closed contacts36 are thus allowed to close. The closing of contacts 36 establishes acircuit from the power source 28 through line 26 to the actuating coilof the time delay relay 2.5 and thence through line 27 to the oppositeside of the power source 28, so the time delay relay 25 is againenergized or reset and opens contacts 29 by moving armature 29a awayfrom contacts 29, thereby to extinguish signal lamp 30. The controlcircuit is thus reset so switch 17-17a can be moved to its auto positionagain and normal operation of the furnace continued.

While we have shown and described a particular embodiment of our glowdischarge detection circuit, it will be obvious to those skilled in theart that various modi- 'fica'tions may be made in our circuit withoutdeparting from our invention in its broader aspects. For instance,

through meohaniea'l-means, such as a slip clutch, to the movabler'ilectrode thereby to afford means, other than the disel'osed voltageresponsive means, for detecting a "stoppage "in the movement or areversal in the direction of movement of the electrode. In such anarrangement, is apparent that the clutch mechanism could be direct lycoupled to a means for resetting the time delay relay. Furthermore, itwill be apparent that while the disclosed invention is particularlyapplicable to the control of the electrode movement of an arc furnace,it is not limited to such an application, but rathermay'be "utilized tocontrol the movement of any apparatus that is designed to move atarelative constant rate in a given predetermined direction, or that isdesigned to move predominantly in a single given direction with onlybrief intervals of reversal in direction or stoppage 'ofmovementoccurring during normal operation.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. -In an automatic control system for an arc furnace having tworelatively movable electrodes, means for applying an arc voltage acrosssaid electrodes, means for providing a reference voltage, means forcomparing the arc voltage with the reference voltage, control meansresponsive to the relative magnitudes of the arc voltage 'and thereference voltage for moving said electrodes tow'ard each other whenthearc voltage is greater than the reference voltage and 'for movingsaid electrodes away from each other when the arc voltage is lower thanthe reference voltage, signal means for indicating an abnormalcondition, time delay means for energizing said signal means following apredetermined time interval after actuation of said time delay means,means dependent upon relative movement of the electrodes for actuatingsaid time delay means and initiating said predetermined time intervalwhen movement of said electrodes toward each other ceases, and meansresponsive to resumption of movement of said electrodes toward eachother for resetting the time delay means,'said time delay means"effecting actuation of said "signal means unless said resetting meansis actuated prior to the expiration ofsaid predetermined time interval.

2. In an automatic control system for an arc furnace having tworelatively movable electrodes, means for applying an arc voltage acrosssaid electrodes, means for providing a reference voltage, means forcomparing thearc voltage with the reference voltage, control meansresponsive to the relative magnitude of the arc Voltage and thereference voltage for moving said electrodes toward each other when thearc voltage is greater than the reference voltage and for moving saidelectrodes away from each other'when the are voltage is lower than thereference voltage, signal means for indicating an abnormal condition,time delay means for energizing said signal means following apredetermined time interval after -actuation of said time delay means,means dcpendent upon movement of the electrodes for energizing said timedelay means when said electrodes move toward each other, said movementdependent means being operable to dc-energize said time delay "means andinitiate "having two relatively -movable electrodes, means forapplyingtan arc voltage a'cross said electrodes, means for providing areference voltage, means "for comparing 'the' 8 arc voltage with thereference voltage, control meaiis responsive to ther'elative magnitudesof the arc voltage and the reference voltage for moving said electrodesto wardca'ch other when the arcvoltage is greater than the referencevoltage and for moving said electrodes :away from each "other when thearc voltage is lower than the reference voltage, signal means forindicating 'an abnormal condition, time delay means for energizings'ai'd signal means following a predetermined time interval afteraetuation of said time delay :means, means elec- 'tr-ical-ly connectedto said control means for actuating 'saiidtitne delay means andinitiating said predetermined time interval when movement of saidelectrodes toward each other -=ceases, and means responsive toresumption of "movement of said electrodes toward *each other forresetting the time delaymeans, said time delay means effecting actuationof said signal means unless said resetting means is actuated prior tothe expiration of sai predetermined time interval.

4. =I-n an automatic control system as defined inc'laim 3, means forvarying the duration of the predetermined timeintervaLand means forvarying the sensitivity of the time delay actuating means.

5. Inan automatic control system for an arc furnace having tworelatively movable electrodes, means -for applying an arc voltage acrosssaid electrodes, means for providing a reference voltage, meansfor'compar'ing the arc voltage with the reference voltage, control means--re sponsive to the relative magnitudes of the arc voltage and the'reference voltage for moving said electrodes towardeac'h other when thearc-voltage is greater'than the reference voltage and for moving saidelectrodes away from 'each'other when the arc voltage-is lower'than thereference voltage, signal means for indicating an abnormal condition,time delay means for energizing said sigrial means following apredetermined time interval "after actuation of said 'time'delay means,voltage responsive means and unidirectional current passing meanselectrically connectedto said control means for controlling said'tim'edelay means, said voltage responsivemeans and "said"unidirectional current passing means being effective to a'ctuatesaid-time delay means and'initiate said predeter- 'niined time intervalwhen the electrodesbecome stationary and to :reset the time delay meanswhen the elect'rodes moveto'wa'rd each'o'ther, whereby said timedelaymeans effects actuation of said s-ignal'means unless said time delaymeans is reset prior to the expiration of said predetermined timeinterval.

,6. In an automatic control system for an arc furnace having tworelatively movable electrodes, means for applying an arc voltage acrosssaid electrodes, means for providing a reference voltage, control meansresponsive to'the relativemagnitudes of -the arc voltage and thereference voltage for moving said electrodes toward each other when the.arc is greater than the reference voltage and for moving saidelectrodes away from each other when the arc voltage is lower than thereference voltage,

said control means including a direct current motor for "driving atleast one of said electrodes, signal means 'for indicating an abnormalcondition, means for energizing that the motor "drives the electrodestogether and current through the voltage're'sponsive means is prevented"when the polarity of the voltage across the motor is such that themotor drives the electrodes apart, said voltage responsive means andsaid unidirectional current passing means being effective to actuatesaid time delay "means and initiate the predetermined time'intervalavhen the electrodes become stationary and tore'set thetimedelay means when the electrodes move toward each other,

whereby said time delay means effects actuation of said signal meansunless said time delay means is reset prior to the expiration of saidpredetermined time interval.

7. In an automatic control system for an arc furnace having tworelatively movable electrodes, means for applying an arc voltage acrosssaid electrodes, means for providing a reference voltage, means forcomparing the arc voltage with the reference voltage, control meansresponsive to the relative magnitude of the arc voltage and thereference voltage for moving the electrodes toward each other when thearc voltage is greater than the reference voltage and for moving theelectrodes away from each other when the arc voltage is lower than thereference voltage, signal means for indicating an abnormal condition, atime delay closing relay having an actuating coil and a pair of normallyclosed contacts, means for energizing the relay and the signal means,means for electrically connecting the normally closed relay contacts andthe signal means in series across the relay energizing means whereby thesignal means is energized following a predetermined time interval afteractuation of said time delay relay, a voltage responsive relay hav ingan actuating coil and a pair of normally open contacts, unidirectionalcurrent passing means, means for electrically connecting the actuatingcoil of said voltage responsive relay in series with the unidirectionalcurrent passing means, means for electrically connecting the normallyopen contacts of the voltage responsive relay in series with theactuating coil of the time delay relay, and means for electricallyconnecting the series con nected unidirectional current passing meansand the actuating coil of the voltage responsive relay to the controlmeans to provide a current through the actuating coil of the voltagerseponsive relay to energize and reset the time delay relay when the arcvoltage is less than the reference voltage and to prevent currentthrough said actuating coil when the arc voltage is greater than thereference voltage, whereby said time delay relay effects actuation ofsaid signal means unless said time delay relay is reset prior to theexpiration of said predetermined time interval.

8. In an automatic control system for an arc furnace having tworelatively movable electrodes, means for ap plying an arc voltage acrosssaid electrodes, means for providing a reference voltage, means forcomparing the arc voltage with the reference voltage, control meansresponsive to the relative magnitudes of the arc voltage and thereference voltage for moving the electrodes toward each other when thearc voltage is greater than the reference voltage and for moving theelectrodes away from each other when the arc voltage is lower than thereference voltage, an electric lamp for indicating an abnormalcondition, a time delay relay for energizing said lamp following apredetermined time interval after actuation of said time delay relay,voltage responsive means for controlling the actuation of the time delayrelay, unidirectional current passing means for controlling theenergization of the voltage responsive means, and means for electricallyconnecting the voltage responsive means and the unidirectional currentpassing means to the control means for controlling said time delayrelay, said voltage responsive means and said unidirectional currentpassing means being effective to initiate said predetermined timeinterval when movement of said electrodes toward each other ceases andto energize and reset said time delay relay when said electrodes movetoward each other, said time delay relay efiecting energization of saidelectric lamp unless said resetting means is actuated prior to theexpiration of said predetermined time interval.

9. In an automatic control system for regulating relative movementbetween two members, control means for effecting movement of one of saidmembers, signal means for indicating an abnormal condition, time delaymeans for actuating said signal means following a predetermined timeinterval after actuation of said time delay means, means responsive torelative movement between said members in a first direction with respectto one of the members for actuating said time delay means to initiatesaid predetermined time interval, and means responsive to relativemovement between the two members in a second direction with respect tosaid one of the members for resetting the time delay means, said timedelay means effecting actuation of said signal means un less said seconddirection of relative movement occurs prior to the expiration of saidpredetermined time interval.

10. In an automatic control system for regulating relative movementbetween two members, an electric motor for effecting movement of one ofsaid members, means for energizing said motor, signal means forindicating an abnormal condition, time delay means for actuating saidsignal means following a predetermined time interval after actuation ofsaid time delay means, and unidirectional current passing meanselectrically connected in series with voltage responsive means andconnected to the motor energizing means for con-trolling said time delaymeans, said voltage responsive means being responsive to a stoppage ofcurrent through the unidirectional current passing means to actuate saidtime delay means to initiate said predetermined time interval, saidvoltage responsive means being responsive to current through theunidirectional current passing means to reset the time delay means, saidtime delay means effecting actuation of said signal means unless currentpasses through the unidirectional current passing means prior to theexpiration of said predetermined time interval.

11. In an automatic control system for regulating relative movementbetween two members, an electric motor for effecting movement of one ofsaid members, signal means for indicating an abnormal condition, a timedelay relay for energizing said signal means following a predeterminedtime interval after actuation of the time delay relay, unidirectionalcurrent passing means electrically connected to the electric motor, avoltage responsive relay electrically connected in series with theunidirectional current passing means for controlling the energization ofsaid time delay relay, said voltage responsive relay being operable tode-energize the time delay relay to initiate said predetermined timeinterval when the unidirectional current passing means blocks currentfrom the voltage responsive relay, said voltage responsive relay beingfurther operable to energize the time delay relay to reset said relaywhen the unidirectional current passing means passes current to thevoltage responsive relay, whereby the time delay relay actuates thesignal means unless the drive motor is energized to cause current topass through the unidirectional current passing means and voltageresponsive relay prior to the expiration of the predetermined timeinterval.

No references cited.

